Wire bonding is the process of making electrical connections in semiconductor components by means of fine metal wire, typically wire with a diameter of from 12 microns to 500 microns. Examples of electrical connections which can be made using wire bonding techniques include connections between the contact surfaces of discrete or integrated chips and the contact leads of their packages, and, in the case of hybrid circuits, the connections between inserted monolithic elements and the film circuit which contains them.
A number of wire bonding techniques have been developed, one which is a microwelding technique using ultrasound. Aluminium wire, in contact with the contact surface to which it is to be bonded, is moved vigorously in the direction of the surface to which it is to be bonded so that its oxide layer breaks open. The wire is then subjected to pressure, and a permanent junction is created between the two materials. Motion of the wire is generated by an ultrasonic transducer excited by an ultrasonic generator to produce high-frequency mechanical vibrations.
In a particular wire bonding process known as wedge bonding, the amount of ultrasonic energy that is supplied depends on a size of the wire used. The ultrasonic energy is directed to the aluminium wire by a special tool known as a “wedge”. The wire is fed through a guide at the bottom of the wedge. When the wedge with the aluminium wire touches the surface to which the wire is to be bonded, movement is stopped. The wire is pressed down with a small defined force, known as the bonding weight and the wire is deformed. This deformation is known as “pre-deformation”. Ultrasonic energy is now switched on and the welding process starts. During this time, the diameter of the aluminium wire is reduced, the actual reduction depending on the size, physical properties and the precise chemical nature of the wire.
It is important in an automatic wire bonding apparatus to have as much control as possible over the process and to be able to determine whether or not a bond has been successfully made. In particular, it is important to be able to ascertain whether a bond has been made successfully at the time of bonding rather than during a subsequent test routine. Because of the very rapid throughput of an automatic wire bonding apparatus, it would be advantageous if the bonding could be monitored immediately at the time of bonding, so that after the formation of an unsatisfactory bond the process can be stopped and the bonding conditions checked to prevent the production of a large number of unsatisfactory bonds, with the consequent wastage of time and expensive components and materials.
Although some existing wire bonding machines can perform certain tests for monitoring bonding failures, these can still be unsatisfactory. Accordingly, it is desired to provide an improved technique for detecting bonding failures.